1. Field of the Invention
The present invention is directed to an X-ray diagnostic installation of the type having an X-ray apparatus for generating X-rays, an X-ray detector for detecting the X-rays and conversion thereof into an electrical signal sequence, an imaging system for processing the electrical signal sequence, and a playback device, and is also directed to a method for the operation of such an X-ray diagnostic installation.
2. Description of the Prior Art
FIG. 1 shows an X-ray diagnostic installation disclosed in German PS 195 27 148 with a first stand 1 to which an X-ray source 2 that generates a cone-shaped X-ray beam 3 is height-adjustably attached. The installation has a second stand 4 to which an X-ray detector 5 is secured such that it is aligned in height with the X-ray 2 such that the X-ray beam 3 is incident on the X-ray detector 5. The output signal of the X-ray detector 5 is supplied to an image computer or imaging system 6. The imaging system 6 can include a computer, transducers, image memories and processing circuits. It is connected to a control monitor 7 for the playback of the acquired X-ray images. A high-voltage generator 8 supplies the X-ray tube of the X-ray source 2 with high-voltage and filament voltage. The imaging system 6 is connected to the other components of the X-ray diagnostic installation via control and data lines 9.
FIG. 2 shows the X-ray detector 5 in a perspective cross-section. The core component of the X-ray detector 5 is a solid-state pixel matrix with line drivers and amplifiers. The solid-state pixel matrix is composed, for example, of a layer with a scintillators 11, for example, of cesium iodide (Csl) that, when irradiated by the X-ray beam 3, emit visible photons into a pixel matrix 12 of amorphous silicon that produces a visible X-ray image. As shown enlarged in FIG. 2, each of the pixels or picture elements of this pixel matrix 12 is composed of a photodiode 13 and a switch 14 that is connected to row lines 15 and column lines 16. The pixel matrix 12 is applied on a glass substrate 10.
All pixels of a row are simultaneously addressed and read out by the line drivers 17. In the simplest case, an image is progressively read out row-by-row. The signals are supplied to a processing circuit 18 in which the signals are processed in parallel in a number of amplifiers, combined by multiplexers, and converted in an analog-to-digital converter (A/D converter) into a digital output signal for further digital processing.
In summary, such known solid-state detectors are based on active readout matrices of, for example, amorphous silicon (a-Si), the image information is converted in an X-ray converter, for example cesium iodide (Csl), is stored in the photodiodes of the matrix as electrical charge and is subsequently read out via an active switch element with a dedicated electronics and converted into a digital signal.
Related technologies likewise employ an active readout matrix of amorphous silicon but employ a converter (for example, selenium) that directly generates electrical charges that are then stored on an electrode. The stored charges are subsequently read out via active switch elements with dedicated electronics and are converted into a digital signal which is further-processed by the imaging system.
An individual picture element (pixel) represents the local X-ray intensity and thus contributes to the overall image. For various reasons, individual pixels, or combinations of pixels, for example rows, columns or clusters, can carry no image information or faulty image information that does not represent the local X-ray distribution.
In general, the individual pixels are independent of one another, so that the signal stored in one pixel has no influence on the signals of the neighboring pixels during the readout process. Under certain circumstances, however, the signal of a pixel or the signals of a combination of pixels may influence the signals of the neighboring pixels during the readout process, and thus faulty signals are supplied at the end of the readout process. These signals are no longer directly representative of the X-ray intensity incident on the respective pixel.
FIG. 3 schematically shows such a configuration. A malfunctioning column 19 at which no signal is present leads to a signal disturbance in the columns lying next to it over a certain length. The disturbancexe2x80x94in length as well as in the amplitude of the signal disturbancexe2x80x94will be less as the spacing of the columns increases from the malfunctioning column 19. This is referred to as xe2x80x9ccometxe2x80x9d 20 due to the shape of the signal disturbance. The disturbance due to the comet artifact arises due to electrical crosstalk by the signal in the malfunctioning column 19. Individual malfunctioning pixels also can be seen in addition to the malfunctioning column 19 and the comet 20.
FIG. 5 shows an exposure of a phantom 22 in which a number of such comet disturbances 23 occur. This is a new phenomenon that occurs in conjunction with the electrical readout process of charges that are generated in solid-state detectors. Since the disturbed region given a comet 23 typically has such a large area that it was not capable of being corrected with standard correction possibilities, detectors having this malfunction were not able to be utilized in the field of medical diagnostics. Due to the large-area disturbance, a correction by interpolation using neighboring, undisturbed pixels is not possible, since such a correction would contain no detail information. Detectors wherein comets arise during the production process or during field utilization, therefore must be discarded.
An object of the present invention is to provide an X-ray diagnostic installation as well as a method for the operation of an X-ray diagnostic installation wherein a correction of comet artifacts can be achieved, so that detectors having this malfunction can be utilized in medical technology.
This object is inventively achieved in an X-ray diagnostic installation wherein the imaging system includes a device for the correction of comet artifacts having an evaluation unit to which a reference value estimator and a detail estimator are connected, these respectively determining a reference value and a detail value at the disturbed locations from the electrical signal sequence, and wherein the output signals of the estimators are supplied to the correction unit, which combines the signals. The invention is based on the perception that the disturbed signal contains residual information, so that the original signal information can be restored by the inventive device.
Inventively, the reference value estimator can be an interpolation unit. The detail estimator can be a selective high-pass filter formed by, for example, a low-pass filter and a subtraction unit.
The above object also is inventively achieved in a method having the steps of implementing a geometrical analysis of the comet artifacts, determining an estimated reference value from the original signal with the assistance of the analysis result, determining an estimated detail signal from the original signal with the assistance of the analysis result, and correcting on the basis of the estimated reference value and of the estimated detail signal.
With the method disclosed herein, the residual information of the signal in the image region defined by the comet is used to restore the signal. As a result, the diagnostically relevant information in the disturbed image region is preserved. The possibility of employing detectors with comets in the field therefore is a considerable advantage.
It has proven advantageous when the geometrical analysis step is a general identification of the comet in a map.
The reference value can be estimated by interpolation from undisturbed signal values outside the comet.
The detail signal can be formed by low-pass filtering in the predominant direction of the comet and subtraction of the filtered result from the original.
The correction can ensue by addition of the estimated reference value and the estimated detail signal.
The geometrical analysis can be implemented online or offline in the context of a calibration.